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Improve correctness of stddev and variance with partial aggregation #23447

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Merged
amitkdutta merged 1 commit into from
Aug 15, 2024
Merged

Improve correctness of stddev and variance with partial aggregation #23447

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8 changes: 7 additions & 1 deletion presto-main/src/main/java/com/facebook/presto/operator/aggregation/AggregationUtils.java
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Original file line number Diff line number Diff line change
Expand Up @@ -217,9 +217,15 @@ public static void mergeVarianceState(VarianceState state, VarianceState otherSt
if (count == 0) {
return;
}
if (state.getCount() == 0) {
state.setCount(count);
state.setMean(mean);
state.setM2(m2);
return;
}
long newCount = count + state.getCount();
double newMean = ((count * mean) + (state.getCount() * state.getMean())) / (double) newCount;
double delta = mean - state.getMean();
double newMean = state.getMean() + delta / newCount * count;
state.setM2(state.getM2() + m2 + delta * delta * count * state.getCount() / (double) newCount);
state.setCount(newCount);
state.setMean(newMean);
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24 changes: 24 additions & 0 deletions presto-tests/src/main/java/com/facebook/presto/tests/AbstractTestQueries.java
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Original file line number Diff line number Diff line change
Expand Up @@ -1237,6 +1237,30 @@ public void testVariance()
assertQuery("SELECT VAR_SAMP(totalprice) FROM (SELECT totalprice FROM orders LIMIT 0) T");
}

@Test
public void testZeroVarianceSamp()
{
assertQuery("SELECT VAR_SAMP(6523763181031200) FROM LINEITEM");
}

@Test
public void testZeroVariancePop()
{
assertQuery("SELECT VAR_POP(6523763181031200) FROM LINEITEM");
}

@Test
public void testZeroStddevSamp()
{
assertQuery("SELECT STDDEV_SAMP(6523763181031200) FROM LINEITEM");
}

@Test
public void testZeroStddevPop()
{
assertQuery("SELECT STDDEV_POP(6523763181031200) FROM LINEITEM");
}

@Test
public void testVariancePop()
{
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